Process for applying cladding of stainless steel on steel base with aluminum bonding layer



R. c. .AVELLONE 3,352,005 R APPLYING CLADDING 0F STAINLESS STEEL BASE WITH ALUMINUM BONDING LAYER Filed Sept. 16, 1963 Nov. 14, 1967 PROCESS ON ST INVENTOR.

RICHARD C. AVELLONE Ila!!! 1/11 4 @m VBW 000 0 1 III/ u Ill 1 O/ H N J] [/11 rill/1 1/11 ATTORNEY United States Patent 3,352,005 PROCESS FOR APPLYING CLADDIN-G 0F STAIN- LESS STEEL 0N STEEL BASE WITH ALUMINUM BONDING LAYER Richard C. Avellone, Mayfield Heights, Ohio, assignor to Republic Steel Corporation, Cleveland, Ohio, a corporation of New Jersey Filed Sept. 16, 1963, Ser. N 309,237 9 Claims. (Cl. 29-484) This invention relates to a process for applying a cladding of stainless steel as a protective metal to a steel base with an aluminum intermediate layer as the bonding agent. More specifically, it relates to a process for cladding a steel base with stainless steel and an intermediate aluminum bonding layer by a rolling operation under controlled temperature and pressure so as to produce at least 3% total reduction in the thickness of the resultant composite laminate.

Two metals which have the same or similar resistances to deformation at the same temperature cannot be effectively bonded by a rolling process. However, if the two materials are heated to different temperatures to obtain differences in the degree of softness or change between their resistance to deformation, bonding between the metals can be obtained. Apparently with metals which have different resistance to deformation at the same temperature, movement between the metal interfaces occurs during rolling and due to the increased friction produced by metal movement, an increase in the atomic attraction forces necessary for good bonding is effected.

The cladding or bonding of stainless steel to a steel base at temperatures below 1400 F. and with reductions of thickness below 45 percent has not been effective. This is very likely due to the fact that their resistances to deformation are not sufficiently different to effect the proper metal movement necessary for increasing atomic attraction forces to give good bonding.

It has now been found that the cladding of stainless steel to a steel base can be effected by the insertion of an aluminum layer, which has a low deformation resistance compared to most metals, between the stainless steel and the base steel and effecting bonding between the dissimilar metals at a temperature in the range of 750-1070 F. under a uniform pressure sufficient to effect at least 2% reduction in the thickness of the aluminum layer, preferably at least 10%, and at least 3% reduction in thickness of the resultant composite laminate. This minimum composite reduction is increased with lower applied temperatures until a minimum of 10% reduction is required at a low temperature of 750 F.

The clad metal can be used on either one or both sides of the base metal with the intermediate layer of aluminum in between the steel base and each layer of stainless steel. The stainless steel advantageously has a thickness of at least 0.001 inch, and the aluminum bonding layer also has a thickness of at least 0.002 inch. The base metal should have a thickness of at least 0.018 inch. The intermediate aluminum layer can be applied as a sheet of aluminum foil or as a layer coated onto the base metal, as in a hot dipping process or vapor deposition coating on the steel or on one side of the stainless steel.

One advantage of the process of this invention is in the relatively low pressure used in the cladding operation. Whereas prior art processes for other types of cladding use excessive pressures which effect 50-90% reduction in the thickness of the laminate, the bonding can be effected by the process of this invention with as little as 3% overall reduction in thickness when the higher temperatures of the specified range are used, and as little as 10% when the lowest temperature of the specified range is used, with proportionate changes in percent reduction as intermediate temperatures are used.

The process of this invention has the advantage that no special preparation or treatment of the metals, such as plating is required. It is only necessary that the metals are first cleaned by conventional methods, placed in their respective positions and spot welded at one end or otherwise maintained in the desired relative positions. The assembled laminate is then placed in or passed through an oven having a reducing atmosphere, such as a disassociated ammonia atmosphere, and heated to a temperature of 750-1070 F. If there are any oxides on the steel base surface this temperature is maintained for a period sufiicient to reduce such oxides, e.g. 1-l5 minutes. At 1070 F. the heating period is generally about 1-5 minutes.

Another advantage of this process is that conventional equipment can be used in the cladding process. For example, the above-described heated assembly can be passed through an ordinary rolling mill to obtain a composite metal reduction of approximately 3%. In prior art processes for other cladding operations, substantially greater reductions are necessary which generally require special rolling equipment. Moreover, the greater reductions esult in greater alteration of the physical properties of the metals. In this invention, the stainless steel cladding metal need not be reduced in the rolling operation, nor need there be any high reduction of the steel base. Consequently, due to the lower pressure required in the process of this invention conventional rolling equipment can be used and the physical properties of neither the cladding metal nor the base metal need be materially affected before commodity fabrication. For the cladding purpose of this invention there is no need nor any advantage in exceeding 30% overall reduction or composite reduction. In this process low or high pressures can be used to clad and roll form a product simultaneously.

In using aluminum foil as the bonding material, it is generally desirable to use a foil having a thickness of 0.002 to 0.06 inch, preferably 0.0040.008 inch. The width and length of the material is limited only by the size of the equipment used in the various steps of the process. Similar thicknesses are appropriate When the aluminum is coated on the base metal or on one side of the stainless steel.

The stainless steel advantageously has a thickness in the range of .006 to 0.025 inch, preferably about 0.006 inch. Particularly desirable types of stainless steel used for this purpose are stainless steels in the 300 and 400 series.

.Where a sheet material is being used as the steel base, this is desirably of a thickness of 0.017 to 0.116 inch,

preferably about 0.075 inch. However, the base can be of any appropriate or desired thickness. A particularly desirable base material is a mild steel. However, the base can be any ferrous metal, other than stainless steel, such as low and high carbon alloy steels, wrought iron, pig

scale on the steel base can be removed by pickling and/or mild sand-blasting.

The furnace used to heat the metal sheets advantageously has a single heating chamber or separate chambers to heat the stainless and mild steels, and to either heat or cool the aluminum sheet. The heating chamber or chambers advantageously has a reducing atmosphere or inert atmosphere toward oxidation of the steel base as well as any other materials that might be sensitive toward the development of films which act as barriers to bonding at the respective temperatures. A particularly suitable reducing atmosphere is a cracked ammonia gas or other gas having a high proportion of hydro-gen therein.

The cooling of the composite product is advantageously conducted in an inert atmosphere, at least until the temperature is lowered somewhat from the furnace temperature. However, rapid cooling in air is generally satisfactory particularly since the surface of the finished product is generally eventually buffed or polished.

The laminated assemblies of this invention can be produced either in batch process or in continuous operation.

In the drawings, FIG. 1 shows a side view of a laminated product of this invention having the stainless steel cladding on both :sides of the steel base.

FIG. 2 is a schematic view showing an arrangement of equipment for continuous operation in the production of steel sheet clad on one side with stainless steel.

FIG. 3 is a schematic view of a similar arrangement of equipment for cladding both sides of the steel sheet with stainless steel.

An important aspect of the invention is that the aluminum layer is not to be held at a temperature of 1070 F. or higher while in contact with the base steel before the pressing operation. The base steel can be slightly above this temperature provided assembly and pressing is effected fast enough to avoid having the aluminum reach 1070 F. or higher for any substantial period. For exarnple if the aluminum is at a temperature of 1075 F. and in contact with the steel base for seconds or more, an alloy of Al-Fe will be formed which inhibits or prevents effective bonding by rolling.

As previously stated the reduction in thickness of the aluminum layer is at least 2% to effect bonding, preferably at least 15%. Excessive amounts of reduction are not necessary to effect good bonding and there is generally no purpose in exceeding 30% reduction of the aluminum layer.

In the batch process, the various sheets are first cleaned, placed in sandwich form in their respective indicated positions with the aluminum and cladding sheets on either one or both sides of the steel base and then tack welded across one end to the steel base so as to prevent any shifting in the relative positions of the sheets. As previously explained, the cladding metal can be put on both sides of the steel base with a layer of aluminum between the base steel and the cladding metal in each case. A single sandwich or pack, or a multiple pack composed of single sandwiches can be placed in the oven by means of a conveyor system or by use of a pusher lance. After the assembled pack has been heated to the desired temperature and cleaned in the reducing atmosphere, the pack is put into a rolling mill with the tack welded end being guided and introduced first. Since the aluminum is reduced in thickness to a higher degree this sheet can be shorter and the proper length provided by the extrusion occurring during the rolling operation.

In continuous operation, the aluminum foil and the stainless steel foil can be fed from coils. The base steel can also be fed from a coil in cases where the thickness is small enough. Otherwise thicker sheets of the base steel can be fed into a system in which the aluminum foil and the cladding metal are being fed in a sandwich arrangement so that the aluminum foil is on one or both sides of the steel base, and the cladding metal is on the outside or opposite side of the aluminum foil from the steel base. The metal sheets in their respective positions can be continuously fed into the heating chamber or chambers of a furnace, individually or combined in a sandwich assembly, at such a rate that the resident time permits the proper removal of barrier films from the metal bonding surfaces, and permits the metal sheets to reach the desired temperature, preferably about 1070 F.

The heated sheets previously assembled, or assembled in sandwich form by joining rolls in the furnace are then continuously fed into a rolling mill which advantageously effects an overall pack reduction of 35%. Generally, with such an overall reduction, there is no deformation of the stainless steel, the aluminum is reduced by about 20% and the steel base is reduced about 2%. The rolled product is then cooled in an inert atmosphere or in air, the edges are subsequently trimmed, and the stainless steel surface is processed by normal means to the desired surface finish.

FIG. 1 shows in an elevational view the laminated structure of a product of this invention in which both sides of steel base 1 are clad with stainless steel 3 with an intermediate aluminum bonding layer 2.

In products having only one side clad with stainless steel, the steel base forms one outer surface of the resultant laminated product and the stainless steel forms the opposite outer surface.

FIG. 2 shows a schematic arrangement of equipment suitable for use for continuous production of the stainless steel clad product produced according to the process of this invention. The various sheet materials are fed from coils. In this arrangement, in which only one side of the steel is to be clad with stainless steel, the steel sheet is fed from coil 6, the aluminum foil 2 is fed from coil 7 and the stainless steel sheet or foil 3 is fed from coil 8. The various sheets, if not previously cleaned, can be passed through a cleaning operation before being fed into the furnace. The three sheets are fed individually through rollers 9 into the furnace 4, through joining rolls 11, out of the furnace through opening 13, through the rolling mill reducing rolls 10, then between cooling air jets 14 before the laminated product is taken up on recoiler 12.

In a corresponding arrangement shown in FIG. 3, a laminated product is produced which has both sides of the steel base coated with stainless steel. In this arrangement, additional coils 7 and '8 are shown positioned on the opposite side of coil 6 from coils 7 and 8 so that aluminum and stainless steel sheets can be fed into joiner rolls 11 in the desired arrangement. In this modification the composite sheet is cooled by passing through the cooling chamber 5, between pressure rolls 11, and then to the recoiler.

The percentage reduction of the composite pack is dependent upon the temperature of the metal sheets at the time of rolling. The percentage reduction can be decreased with an increase in the temperature of the metal sheets. It is only necessary that the minimum specified percent of reduction be effected and that it be etfected within the temperature range indicated. Actually since the reduction is of such a small amount and the temperatures much lower than used in other cladding operations, the reduction and the temperature conditions can be met very easily. Generally one pass through the rolling mill effects the desired reduction and a very short heating period brings the metal to the desired temperature range.

The invention is best illustrated by the following examples. The examples are not intended to limit in any way the scope of the invention nor the manner in which it can be practiced. In the examples and throughout the specification, reference to parts and percentages are to parts and percentages by weight respectively.

Example 1 Sheets of a plain steel having 0.3 percent carbon, having a width of six inches, length of 22 inches and thickness of 0.075 inch, aluminum foil having a length of 18 inches and the same width as the steel sheet and a thickness of 0.005 inch, and sheets of a 300 stainless steel having a thickness of 0.006 inch and the same width and length as the steel sheet are each cleaned by vapor degreasing to remove organic contaminations. Oxide scale on the steel is removed by pickling; stains on the stainless steel are also removed by a light pickle dip; and stains on the aluminum foil are removed by wire brushing. Then the aluminum foil is placed over the steel base and the stainless steel sheet place-d over the aluminum foil so that the edges of the three sheets make the respective edges of the composite sandwich or pack. The three sheets are then spot welded to each other across the width at one end of the pack. This pack is placed in an electrically heated oven through which carcked ammonia gas is passed at a rate of 0.50 cubic feet per minute for a period of 5 minutes. When the thermocouples indicate that the pack temperature is at 1050 F., the oven is rolled closed to the rolls of a 2-high, 14 inch diameter, reversible Loewy mill (or the rolls of a 2-high, 5 inch diameter reversible Stanat rolling mill) which are flooded with jets of reducing gas. The pack is then pushed by means of a lance inserted at the rear of the furnace so that the tack welded pack end leaves the front of the furnace and passes into the rolling mill. The roll openings are adjusted to obtain an overall pack reduction of from 3 to 5% and the rolls are operated at a speed of to 80 feet per minute. After one pass through the rolling mill, the pack is cooled. In the rolled product, the stainless steel sheet has no deformation whereas the aluminum is reduced by and the steel base by 2%. The edges, front and back of the cooled pack are trimmed and the stainless steel surface given a light pickle treatment and then buffed to the desired surface finish. The metallurgical bond between the various sheets is good as shown by Nitol etching, which also shows the absence of alloy formation. The metals cannot be easily separated by pliers test, 180 degree bend test, lock seam test, or Olsen button forming test.

In the pliers test, one corner of a clad sample is placed in a vice and the sample bent back and forth repeatedly until the steel breaks. A long-nosed pliers is used to pull or peel the stainless steel from the steel base. The bond is evaluated as follows according to the force required to pull the metals apart:

1 Poor b0nd.If metals easily pulled apart.

Fair b0nd.If moderate force pulls them apart. Good bond.If considerable force is required. Excellent b0nd.If very difficult to separate or if one of the metals, usually the aluminum, rips upon pulling.

In the 180 bend test, the sample is folded 180 degrees. If there is no de-lamination, the bond is good.

Standard forming tests also used to evaluate the bond are the Olsen button test, the lap (or lock) seam test, the texturing test, and the weather seal channel section test.

Example 11 Similar results are obtained when the procedure of Example I is repeated using, as the base instead of the plain steel of Example I, a steel of 02 carbon, a steel of.0.3% carbon, 1.0% Ni, 0.5% Mo and 0.9% Cr and a steel of 0.5% Ni, 1% Mo and 2.2% Cr, respectively.

Example III The procedure of Example I is repeated with excellent results using a 302 stainless steel, a pure aluminum foil, a mild steel base, a pack temperature of 850 F. for 10 minutes and a rolling speed of feet per minute to give the following reductions:

6 Example IV The procedure of Example I is repeated except that the steel sheet is clad on both sides by the stainless steel sheets with an aluminum foil layer between the sheet base and the respective stainless steel sheets. A heating of 700-750" F. for 10 minutes is used and a rolling rate of 25 ft. per minute. The overall pack reduction is 14%. In this case also, the various sheets have good bonding to the adjacent sheets.

Example V The procedure of Example I is repeated using a 302 stainless steel and in place of the steel base of Example I, the following types of steel respectively: (a) SAE 1008 (Al-killed deep drawing quality); SAE 1010; and SAE 1020.

and good bonding of the sheets is obtained.

Example VI The procedure of Example I is repeated with similar results using in place of the 300 stainless steel of Example I, the following respective stainless steels: (a) an AISI type 200 stainless steel; and (b) an AISI type 400 stainless steel.

Example VII With equipment arranged as shown in FIG. 2, a single clad steel is made using a coil of a plain carbon steel having 0.2% C in a 24 inch Width and a thickness of 0.017 inch, a coil of aluminum foil of corresponding width and a thickness of 0.005 inch, and a coil of type 400 stainless steel having a corresponding width and a thickness of 0.006 inch. The respective sheets in these coils are prer cleaned as in Example I. The speed of the sheet passing through the furnace is adjusted so that the temperature reaches approximately 1070 F. by the time it has reached the pressing and the pressure of the rolls is adjusted to give a 34% reduction in the ultimate product. The cooling is effected by blowing air on the composite sheet through cooling air jets 14. Upon finishing the edges and bufiing the stainless steel surface as in Example I, the resultant composite sheet shows a reduction of 3.4% and excellent adherence of each sheet to each adjacent sheet. Similar results are obtained when the steels of Example II are substituted for the above 0.2% carbon steel.

Example VIII The procedure of Example VII is repeated using an additional coil of aluminum foil and stainless steel in an arrangement as shown in FIG. 3. Here again excellent adherence of each sheet to each adjacent sheet is obtained in the resultant product.

Example IX The procedure of Example IV is repeated except that in place of the aluminum foil of that example, a steel sheet is used on which an aluminum layer has been deposited by vapor deposition of 0.004 inch on each side of the steel sheet. Upon processing as in Example IV, excellent adherence of the stainless steel is obtained.

Example X Example XI The procedure of Example VIII (clad on both sides) is repeated except that coils 7 and 7' of aluminum foil are omitted and the steel base of coil 6 is passed through a In each case about 14% overall reduction is effected hot dip aluminum bath before being fed into rollers 9. In this way a deposit of 0.002 inch of aluminum on each side of the steel sheets is effected before the rolling operation. Examination of the resultant product shows excellent adherence of the stainless steel to the steel base.

Example XII The procedure of Example I is repeated except that in place of using the separate aluminum foil, the steel sheet has previously been laminated by friction rolling of an aluminum sheet on the steel sheet to give a 0.005 inch layer of aluminum on one side of the steel sheet. Upon processing, similar results are obtained as in Example I.

Example XIII The procedure of Example 1V is repeated except that in place of using the separate aluminum foil, the steel sheet has previously been laminated by friction rolling of an aluminum sheet on the steel sheet to give a 0.005 inch layer of aluminum on both sides of the steel sheet. Upon processing, similar results are obtained as in Example IV.

Example XIV The procedure of Example XII is repeated except that instead of depositing the aluminum layer on the steel base the corresponding thickness of aluminum is deposited on one side of the stainless steel sheet and this sheet is placed against the steel base with the aluminum coating toward the steel base. Similar results are obtained as in Example XII.

Any type of steel can be used as the steel base. The type of steel used is determined by the cost and the end use of the clad product. Advantageously, carbon steels and low alloy steels are used. Typical illustrations of such preferred steels are given on P. 866 of Langes Handbook of Chemistry, 9th Edition (1956) published by Handbook Publishers, Inc. Such steels generally have at least 0.1% carbon and no more than 10%, advantageously no more than of any alloying metal. However, as indicated above, other ferrous metals can be used including wrought iron, pig iron, etc.

The stainless steels that can be used in the practice of this invention as the cladding material can be defined as containing at least 12% iron and at least 12% of at least one metal selected from the class consisting of chromium, nickel and molybdenum, with at least 85 of the composition being of metals of the class consisting of iron, chromium, nickel and molybdenum. Typical compositions of such stainless steels are given on pp. 864-865 of the aforementioned handbook.

While certain features of this invention have been described in detail with respect to various embodiments thereof, it will, of course, be apparent that other modifications can be made within the spirit and scope of this invention and it is not intended to limit the invention to the exact details shown above except insofar as they are defined in the following claims:

The invention claimed is:

1. A process for cladding a stainless steel sheet on a steel base comprising the steps of applying a stainless steel sheet against a steel base with an intermediate independent sheet of alumium having a thickness of 0.002-0.06 inch so that the independent aluminum sheet is in direct intimate contact with each of the adjacent sheets, maintaining the respective sheets at a temperature of at least 750 F. and below the temperature at which Al-Fe alloy will be formed in the period of contact, and pressing said sheets. against each other under sufi'icient pressure to effect a reduction of 2-30 percent in the thickness of the aluminum sheet.

2. A process for cladding a stainless steel sheet on a steel base comprising the steps of applying a stainless steel sheet against a steel base with an intermediate independent thin sheet of aluminum so that the independent aluminum sheet is in direct intimate contact with each of the adjacent sheets, maintaining the respective sheets at a temperature of approximately l050 F., and pressing said sheets against each other under suflicient pressure to effect a reduction in the overall thickness of the composite of approximately 3-5 percent.

3. The process of claim 1 in which said stainless steel sheet has a thickness of 0006-0025 inch, and said steel base has a thickness of at least 0.017 inch.

4. The process of claim 2 in which said stainless steel sheet has a thickness of 0.006-0.025 inch, said aluminum sheet has a thickness of 0002-0706 inch and said steel base has a thickness of at least 0.017 inch.

5. The process of claim 1 in which said stainless steel contains at least 12% iron and at least 12% of at least one metal selected from the class consisting of Cr, Ni and Mo, and at least of the stainless steel composition comprises metals selected from the class consisting of Fe, Cr, Ni and Mo.

6. The process of claim 5 in which said steel base is a steel containing at least 0.01% carbon and no more than 10% of any alloying metal.

7. The process of claim 1 in which said steel base is a steel containing at least 0.01% carbon and no more than 10% of any alloying metal.

8. The process of claim 4 in which said steel base has a thickness of 0.017-0.116 inch.

9. The process of claim 1 in which said temperature is approximately 1050 F. and the overall reduction in thickness is approximately 3-5 percent.

References Cited UNITED STATES PATENTS 2,908,073 10/1959 Dulin 29 494 X 3,078,563 2/1963 Goulel et al 29 497.5 X 3,093,885 6/1963 Morrison et al. 29196.2 X 3,132,418 5/1964 Fulford 29 497.5 X 3,173,202 3/1965 Farber 29 3975 X 3,195,991 7/1965 Stern et al. 29-4975 X 3,210,840 10/1965 Ulam 29 497.5X

JOHN F. CAMPBELL, Primaly Examiner. R. F. DROPKIN, Examiner. 

1. A PROCESS FOR CLADDING A STAINLESS STEEL SHEET ON A STEEL BASE COMPRISING THE STEPS OF APPLYING A STAINLESS STEEL SHEET AGAINST A STEEL BASE WITH AN INTERMEDIATE INDEPENDENT SHEET OF ALUMINUM HAVING A THICKENSS OF 0.002-0.06 INCH SO THAT THE INDEPENDENT ALUMINUM SHEET IS IN DIRECT INTIMATE CONTACT WITH EACH OF THE ADJACENT SHEETS, MAINTAINING THE RESPECTIVE SHEETS AT A TEMPERTURE OF AT LEAST 750*F. AND BELOW THE TEMPERATURE OF WHICH AL-FE ALLOY WILL BE FORMED IN THE PERIOD OF CONTACT, AND PRESSING SAID SHEETS AGAINST EACH OTHER UNDER SUFFICIENT PRESSUE TO EFFECT A REDUCTION OF 2-30 PERCENT IN THE THICKNESS OF ALUMNINUM SHEET. 